A great point came up in the Wrist UX panel I was on yesterday at SXSW, prompted by a user question about what makes the best watch app.
The best Apple Watch apps in my mind are the ones that include the most useful and frequently relevant complications. The watch face itself is the best piece of real estate on the watch. That's park avenue. It's what people will see all the time. The complications that inhabit it are the fastest way for users to launch your app. Having a great complication puts you in a prime position to have users interact frequently with your app while inherently giving them quick, timely updates at a glance. It’s an amazing feature for users, and the most rewarding should you get it right.
Designing a great complication is actually very hard to do. For complications to be used frequently they need to be something that a user would keep on their watch face at all times, meaning they should always have something relevant to show.
Weather and fitness apps are great use cases, and I think the biggest reason why is the data they present you with is easy to take action on. If your activity ring or dashboard wheel isn’t full when you check the time before dinner, you might choose to go for a run. If you glance at your watch while getting ready in the morning and notice the temperature, you’ll remember to bring a coat. The fact that you notice these bits of information while doing something unrelated like checking the time helps you in a way you didn’t expect it to.
A complication that tracks a flight or a package is also very useful, but it's only relevant for a limited portion of time. These are the types that power users might set on a secondary watch face to use occasionally. Ultimately, I hope that Apple will eventually allow complications to be more dynamic based on context, but for now the best complications are the ones that are literally always relevant to the user.
Complications are also very difficult to design for because they have to provide relevant data without needing to be updated frequently. Generally speaking, a complication can only update itself once every 10 minutes. If your concept for a complication requires more frequent updates than that, then you may have to go back to the drawing board.
Getting the complication right is the key to unlocking a huge amount of potential on the Apple Watch. Once you get your one main use case nailed on the watch app, I would focus the rest of your energy on designing a great complication. It's difficult to do, and competition for space on the watch face is stiff, but when a user chooses to place your complication on their watch face that's when you know you've built a great watch app.